Reading JSON Files with Pandas: Handling Errors like a Pro

Reading JSON Files with Pandas: Handling Common Errors

JSON (JavaScript Object Notation) is a widely used data format for storing and exchanging data between applications. JSON files can be easily parsed and manipulated using programming languages like Python, especially with the help of specialized libraries like Pandas.

However, working with JSON files can be tricky, especially if they contain some common errors. In this article, we will explore two common scenarios when reading JSON files with Pandas, and the solutions to resolve them.

Scenario 1: Multiple Top-Level JSON Objects in File

The first common error occurs when a JSON file contains multiple top-level objects, which means they are not enclosed in square brackets and separated by commas. Let’s say we have a sample JSON file containing two objects:

{"name": "John", "age": 25}
{"name": "Jane", "age": 30}

If we try to read this file using Pandas’ read_json function, we get the following error:

ValueError: Trailing data was found after reading from file path/to/json_file.
Expecting object, found '
{"name": "Jane", "age": 30}
'. This error occurs because the read_json function expects the file to contain a single JSON object.

To resolve this issue, we need to combine the multiple objects into a list. One way to do this is by using the json module in Python:

with open(‘path/to/json_file’) as f:

    data = [json.loads(line) for line in f]

This code reads the file line by line and loads each line as a separate JSON object, which is appended to a list. Alternatively, Pandas also provides a solution to this issue by using the lines=True argument in the read_json function:

df = pd.read_json('path/to/json_file', lines=True)

This tells Pandas to treat each line of the file as a separate JSON object and read them as a list.

Scenario 2: Newline Characters in Data Causing Error

Another common error occurs when there are newline characters (n) in the data of a JSON file.

Consider the following example:

{"name": "JohnnDoe", "age": 25}

When we try to read this file using Pandas’ read_json function, we get the following error:

ValueError: Trailing data was found after reading from file path/to/json_file. Expecting ',' delimiter: line 1 column 15 (char 14)

This error occurs because the n character is not a valid character in JSON syntax, and Pandas fails to parse the data correctly.

To resolve this issue, we can use the lines=True argument with the read_json function, as discussed earlier. Another solution is to replace the newline characters with valid escape sequences, such as n:

with open(‘path/to/json_file’) as f:

    data = f.read().replace('n', 'n')
    df = pd.read_json(data)

This code replaces all occurrences of n with n, which is a valid escape sequence in JSON syntax.

Solutions to Trailing Data Error

In both of the above scenarios, the main issue is the trailing data error, which occurs when there are additional characters or data after the proper JSON object in the file. To resolve this issue, we can use the methods discussed earlier, which include combining multiple objects into a list or using the lines=True argument with the read_json function.

However, there are a few additional points to note:

• The first solution of combining multiple objects into a list may not work if the file is very large, as it may consume excessive memory.
• The second solution of using the lines=True argument works well with files that contain single-line JSON objects.
• However, it may not work correctly with files that contain JSON arrays or more complex JSON structures.
• In some cases, the trailing data error may occur because there are extra characters such as white spaces, tabs or commas after the last closing bracket of the JSON object.
• To resolve this, we can remove any unnecessary characters at the end of the JSON object.

Conclusion

In this article, we discussed two common scenarios when working with JSON files using Pandas and the solutions to resolve them. We learned that the trailing data error is a common issue that can occur due to various reasons, such as multiple top-level JSON objects or newline characters within a JSON value.

We also discussed the solutions to this issue, including combining multiple objects into a list, using the lines=True argument with the read_json function, and removing unnecessary characters at the end of the JSON object. By understanding these common errors and their solutions, we can better handle JSON files during data analysis and manipulation using Pandas.

Example of Multiple JSON Objects in File Causing Error

Consider the following sample JSON file:

{"name": "John", "age": 25}
{"name": "Jane", "age": 30}

If we try to read this file using Pandas’ read_json function without any modifications, we get the following error:

ValueError: Trailing data was found after reading from file sample.json. Expecting object, found '
{"name": "Jane", "age": 30}
'.

This error occurs because the read_json function expects a single JSON object in the file, but there are two top-level JSON objects separated by a newline character. To resolve this issue, we can modify the file to contain a list of JSON objects:

[
{"name": "John", "age": 25},
{"name": "Jane", "age": 30}
]

Now, if we read this modified file using Pandas’ read_json function, it will correctly parse the data into a DataFrame:

df = pd.read_json('sample_modified.json')
print(df)

Output:

   name  age
0  John   25
1  Jane   30

Alternatively, we can also read the original file as-is by using the lines=True argument with the read_json function:

df = pd.read_json('sample.json', lines=True)
print(df)

Output:

   name  age
0  John   25
1  Jane   30

This argument tells Pandas to read each JSON object as a separate line, which works well for files containing multiple JSON objects separated by a newline character.

Example of Newline Characters in Data Causing Error

Consider the following sample JSON file:

{"name": "JohnnDoe", "age": 25}
{"name": "Jane", "age": 30}

If we try to read this file using Pandas’ read_json function without any modifications, we get the following error:

ValueError: Trailing data was found after reading from file sample.json. Expecting ',' delimiter: line 1 column 15 (char 14)

This error occurs because the n character in the first JSON object is not a valid character in JSON syntax.

To resolve this issue, we can modify the file by replacing the n character with a valid escape sequence like n:

{"name": "JohnnDoe", "age": 25}
{"name": "Jane", "age": 30}

Now, we can read this modified file using Pandas’ read_json function:

df = pd.read_json('sample_modified.json')
print(df)

Output:

        name  age
0  JohnnDoe   25
1       Jane   30

Alternatively, we can also modify the code to read the original file as-is and replace the newline characters with valid escape sequences:

with open(‘sample.json’) as f:

    data = f.read().replace('n', 'n')
    df = pd.read_json(data)
print(df)

Output:

        name  age
0  JohnnDoe   25
1       Jane   30

This code reads the file using Python’s built-in open function, replaces the newline characters with n using the replace function, and then passes the modified data to Pandas’ read_json function.

Conclusion

In this section, we provided detailed examples of the two common scenarios when reading JSON files using Pandas and the solutions to resolve them. We saw that combining multiple JSON objects into a list or using the lines=True argument with the read_json function can resolve the issue of trailing data error that occurs when there are multiple top-level JSON objects in a file.

We also saw that replacing newline characters with valid escape sequences or modifying the file to remove the newline characters can resolve the issue of trailing data error that occurs due to newline characters within a JSON value. By understanding these examples and their solutions, we can handle JSON files with ease when working with Pandas.

Resolving Multiple JSON Objects in File Causing Error

In addition to the solutions we discussed earlier, we can also use an editor such as Notepad++, Sublime Text, or VS Code to combine multiple top-level JSON objects into a list. This solution is relatively easy and straightforward, especially for small JSON files.

Here’s how we can do it using Sublime Text:

• Open the JSON file in Sublime Text
• Use the Find & Replace function (Ctrl+H) to replace all new-line characters with a comma (,) and an additional new-line character as follows:

Find: n

Replace: ,n

• Wrap the entire content with a square bracket [ at the beginning and ] at the end of the file.
• Save the modified JSON file.

This process will result in a list of all JSON objects enclosed in square brackets and separated by commas. Now, if we read this modified file using Pandas’ read_json function, it will correctly parse the data into a DataFrame as demonstrated earlier in the article.

Another solution to this issue is to use the lines=True argument with the read_json function, which tells Pandas to treat each line of the file as a separate JSON object and read them as a list.

Resolving Newline Characters in Data Causing Error

We discussed a solution to this issue earlier in the article, which involves replacing the newline characters with valid escape sequences (n) and then passing the modified data to Pandas’ read_json function. However, this solution may not always be suitable, especially for large JSON files.

A modified solution that works better for large JSON files is to use the lines=True argument with the read_json function and then remove the newline characters from the data columns. Here’s how we can do it:

# read file using lines=True argument
df = pd.read_json('sample.json', lines=True)
# remove newline characters from data columns
df = df.replace('n',' ', regex=True)
print(df)

This code first reads the file using the lines=True argument with the read_json function, which ensures that each line is treated as a separate JSON object. Then, we use the replace function to remove the newline characters from all data columns.

The replace function is called with two arguments, the first is the string of characters we want to replace (here, n), and the second is the string to replace it with (here, space). The regex=True argument tells the function to use regular expressions to search for the n character.

Output:

        name  age
0  John Doe   25
1       Jane   30

Now, the DataFrame is correctly parsed without any error caused by newline characters in the data.

Conclusion

In this section, we provided additional solutions to the common issues that can occur when reading JSON files using Pandas, specifically regarding multiple top-level JSON objects in a file and newline characters in the data. We saw that combining multiple top-level JSON objects into a list using an editor or using the lines=True argument with the read_json function can help resolve the issue of trailing data error that occurs in the first scenario.

Additionally, we saw that using the lines=True argument coupled with removing the newline characters from the data columns can help resolve the issue of trailing data error that occurs in the second scenario. By understanding these solutions, we can more efficiently handle JSON files containing these common issues in data analysis using Pandas.

Conclusion

In this article, we discussed the common errors that can occur when reading JSON files using Pandas, along with their solutions. These two scenarios included multiple top-level JSON objects in a file and newline characters in the data.

To resolve these issues, we explored solutions such as combining objects using an editor or using the ‘lines=True’ argument with the ‘read_json()’ function with modifications to remove the newline characters. These solutions can ensure that the trailing data error does not occur and guarantee error-free parsing of the DataFrame.

The ability to understand these common issues and their solutions is essential for anyone working with JSON files in data analysis using Pandas.